1. Field of the Invention
The present invention relates to a golf ball having excellent adhesion between a ball body and a coated film as the outermost surface layer of the golf ball, and a method of producing the same.
2. Description of the Prior Art
Conventionally, a golf ball is made by coating the surface of a ball body with a white paint or clear paint to form a coated film. The coated film improve cosmetic appearance of the ball and prevent stain from sticking on the surface thereof.
Since a golf ball is used under very severe conditions such as being subject to a great deformation upon impact of a shot and abrasion by sand or other objects, thus strong adhesion is required between the coated film and the ball body.
The surface of the ball body is treated before coating to increase the adhesion between the coated film and the ball body. For a golf ball having an ionomer resin cover as the surface layer of the ball body, such treatments are proposed as flaming process wherein the surface of the ball body is oxidized by burning with flame and plasma treatment process (Japanese Unexamined Patent Publication No.S60-215374). In the case of a golf ball having a rubber layer on the surface of the ball body, such treatments are proposed as immersion active chlorine water (Japanese Unexamined Patent Publication No. 5-317459), immersion in a hydrochloric acid water containing a hypochlorite (Japanese Unexamined Patent Publication No. 4-277533) and immersion in sulfuric acid (Japanese Unexamined Patent Publication No. 9-276447).
Recently lost ball market has been thriving because lost balls is widely used on practice courses. Under these circumstances, longer available period of golf balls and stronger adhesion between the golf ball body and the coated film are required. Strong adhesion is required not only when the golf ball is under dry condition but also under wet conditions such as in the case of falling into water.
However, all treatments in the prior art set forth aim at increasing the physical bonding between the ball body and the coated film by roughing the surface of the ball body, and cannot satisfy the requirement for strong adhesion imposed today, particularly the requirement for strong adhesion under wet conditions.
Moreover, the treatments that employ chlorine water, hydrochloric acid water containing a hypochlorite or sulfuric acid, and the flaming treatment require facilities for protecting operators and therefore have disadvantage in terms of cost. The plasma treatment has also disadvantage also in terms of cost for plasma generating facilities.
Japanese Unexamined Patent Publication No. 10-165886 proposed a method of treating a ball body surface with a diluted solution of a silane coupling agent. The surface treatment can be accomplished by using a simpler and cheaper facility, because it suffices only to dip the ball body in the diluted solution of the silane coupling agent.
However, the surface treatment with a silane coupling agent needs a drying process before application of paint to the treated surface. The drying process is necessary for the solvent used in diluting the silane coupling agent to evaporate almost completely. It is because, when the ball body is coated with the paint while the solvent remains on the surface thereof, the paint cannot dry satisfactorily and paint adhesion becomes weak.
The treatment with silane coupling agent has another problem associated with a batch operating process. The batch operating process is conducted in such a manner that an appropriate number of ball bodies are simultaneously subjected to the surface treatment and then all the surface-treated ball bodies are transferred to a coating process that follows. In the batch operating process, ball bodies that have completed the surface treatment are transferred to the subsequent coating process after being left for about one hour. The interval between the surface treatment and the coating process, i.e. about one hour, is used for vaporizing the diluting solvent, which is convenient in the manufacturing processes.
The silane coupling agent used in the above publications is a monomer silane coupling agent represented by the general formula (RO)axe2x80x94Si(X)b, wherein R is an alkyl group, and X is a haloalkyl group, an amino-substituted alkyl group, a (meth)acryl or (meth)acryloxy-substituted alkyl group, a mercapto-substituted alkyl group, or a glycidyl-substituted epoxy group. Silane coupling agents such as vinyltrichlorosilane, xcex3-chloropropyltrimethoxysilane, vinyltris(xcex2-methoxyethoxy)silane, N-xcex2(aminoethyl)xcex3-aminopropyltrimethoxysilane and xcex3-glycidyloxypropyltrimethoxysilane are all highly volatile.
When left for one hour after the surface treatment, not only the solvent but also the silane coupling agent itself volatilizes so that sufficient part thereof does not remain on the surface of the golf ball body. In this case, the expected effect of improving the adhesion of the coated film cannot be obtained despite the surface treatment with the silane coupling agent. Although the drying process may be eliminated by using the silane coupling agent without diluting, a simplified surface treatment process such as immersion and spray coating cannot be employed due to high viscosity of the silane coupling agent. Taking productivity and convenience into consideration, a sequential system where individual golf balls that have been surface-treated are coated one by one is not suitable for manufacturing golf balls.
It is an object of the present invention to provide a method of producing golf balls improved the adhesion between a ball body and a coated film on the ball body without the need of expensive and large scale facilities. The method is applicable to the batch operating process. Another object of the present invention is to provide a golf ball thus produced.
The inventors have found that the volatility of a silyl compound is associated with the molecular weight and the number of silyl groups contained in one molecule, and completed the present invention.
The inventive method comprises the steps of: applying an adhesion enhancing agent containing a silyl component and a solvent to a surface of a ball body; drying the coated surface to form an adhesion enhancing layer having the silyl component; and applying a paint to the adhesion enhancing layer. The silyl component is capable of remaining 30 weight % or more after an elapse of one hour at a temperature of 25xc2x0 C. It is preferred that the silyl component include at least 50% by weight of a silyl compound (P) with respect to a total weight of the silyl component, the silyl compound (P) is represented by the following formula (1) and has a weight-average molecular weight of 300 or higher: 
(wherein R1 and R2 are the same or different and each represents a saturated alkyl group having 1 to 3 carbon atoms; X and Y each represents an organic group having an amino group, an imino group or an epoxy group in a molecule; xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d are integers of 2 or 3, and xe2x80x9cbxe2x80x9d and xe2x80x9cdxe2x80x9d are integers of 0 or 1, with a proviso that a, b, c and d satisfy the following relations: a+b=3 and c+d=3; and xe2x80x9cAxe2x80x9d is a divalent residue containing at least one selected from the group consisting of an amide group, an imino group, a tertiary amino group and an alkylene group).
According to the present invention, there is also provided a golf ball which comprises a ball body, an adhesion enhancing layer containing a silyl component formed on the ball body, and a coated film formed on the adhesion enhancing layer. The silyl component is capable of remaining 30 wt % or more after an elapse of one hour at a temperature of 25xc2x0 C.
The present invention also provides a golf ball which comprises a ball body, an adhesion enhancing layer formed on the ball body which contains a silyl compound (P) represented by the following formula (1) and having a weight-average molecular weight of 300 or higher, and a coated film formed on the adhesion enhancing layer. 
(wherein R1 and R2 are the same or different and each represents a saturated alkyl group having 1 to 3 carbon atoms; X and Y each represents an organic group having an amino group, an imino group or an epoxy group in a molecule; xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d are integers of 2 or 3, and xe2x80x9cbxe2x80x9d and xe2x80x9cdxe2x80x9d are integers of 0 or 1, with a proviso that a, b, c and d satisfy the following relations: a+b=3 and c+d=3; and A is a divalent residue containing at least one selected from the group consisting of an amide group, an imino group, a tertiary amino group and an alkylene group).